Heat stress in the workplace can cause death or serious harm, routinely causes workers to experience a range of heat-related disorders, and is implicated as the contributing cause to injuries and accidents. Heat stress has been part of many industrial processes from the time the process was created, and it affects people at work and play. Because it is so common, it is frequently discounted as an important hazard. The exposure assessment method developed in the early 1970s uses an index of the environment called the wet bulb globe temperature (WBGT), with a threshold value that decreases with increasing metabolic rate. The threshold curve establishes a sensitive measure for chronic, steady exposures to heat stress while wearing cotton work shirt and pants. Short-term exposures and protective clothing greatly limit the current WBGT method to the point that it is often not applicable. The research project will make the assessment process relevant to contemporary occupational heat exposures. This will be accomplished by providing (1) WBGT adjustments for different categories of clothing, (2) information on the heat exchange properties of the clothing for use in short-term evaluation schemes, and (3) an empirical exposure limit based on clothing and WBGT. In the first year, the WBGT adjustments and heat exchange characteristics will be established for five clothing ensembles by looking for a range of environments in which subjects representing typical workers can just maintain body core temperature at a moderate rate of work. The second year will closely examine the contribution of light, moderate and heavy work on the same factors for a representative set of clothing. In the third year, the emphasis will be on high heat exposures that result in limited work times. This last year will be the test of the heat exchange model and the source of data for the empirical exposure limits.